European Patent Application No. 0 461 119 (corresponding to International Patent Publication No. WO 09/10195) describes a heterodyne interferometer in which a laser diode is used as the radiation source. The injection current of the laser diode is periodically modulated using a saw tooth or triangular modulation signal, for example. Injection current modulation results in periodic changes in the frequency of the optical beam produced by the laser diode, which is split by a beam splitter into two beam paths. The optical measured route is in the first beam path, and an optical monostable element is provided in the other beam path. Due to the periodic modulation, a frequency difference, corresponding to the heterodyne frequency, always appears between the two beam paths. The two partial beams are brought to a beam receiver for interference after traveling through the measured route. The measured signal generated at the beam receiver has a sinusoidal signal shape. The frequency of the measured signal is equal to the heterodyne frequency. The information about distance, angle or velocity is contained in the phase angle of the measured signal with respect to a reference signal. The reference signal can also be made available optically in an optical reference route. In this case, the two partial beams are brought to an additional beam receiver for interference after traveling through the optical reference route. The optical reference route may be omitted if the reference signal is derived electrically from the modulation signal of the laser diode.
European Patent Application No. 0 729 583 describes a device for measuring the phase difference between two electric signals occurring in a heterodyne interferometer. Each predefined edge of a first signal and each predefined edge of a second signal trigger counting sequences that allow the determination of the phase difference from multiples of 360.degree. and high-resolution measurement of the angle from 0.degree. to 360.degree.. No instructions are given regarding the pre-processing of the signals sent to the known device.
The object of the present invention is to provide a circuit for pre-processing signals occurring in a heterodyne interferometer.